In many applications it is necessary to use a rotational angle transducer to obtain information about the absolute angular position of a shaft, and in some cases in this context the information must in turn be available directly after a system is switched on. An example of such an application is, for example, the throttle valve shaft of a motor vehicle in which the use of an incremental position sensor is becoming widespread since such a sensor has to carry out reference travel when the system starts and in this context it moves against a mechanical end position. In the case of a throttle valve shaft, this would lead, in one of the end positions, to the engine stalling, and in the other end position to an open throttle position of the throttle valve when the engine has already started. Reference travel before the starting of the engine would lead to a time delay which is not accepted by the customer. Incremental position sensors are also not optimal for safety considerations because there is the risk that during operation faults on the lines, such as can occur for example due to the ignition, can result in a movement of the monitored shaft being simulated, which can lead to considerable functional problems.
The previously mentioned objections relate, for example, to the throttle valve shaft of a motor vehicle which has already been mentioned. In such a throttle valve shaft, for example what are referred to as magneto-resistive sensors (see for example DE 197 31 555 A1) or sensors which operate with inductive methods are used. Basically, although these are suitable for meeting the requirements, the manufacturing costs are at a relatively high level, in particular also because complex digital signal processing is necessary to linearize the output signals of the sensors. A known permanent-magnetic sensor is also the analogue Hall sensor. In such a sensor the signal conditioning is considerably simpler, and with skillful configuration there is even no need at all for digital signal processing. However, the interaction of a Hall sensor with a magnetic field is subject to disadvantages because the linear measuring range of a Hall sensor is not sufficient for simple magnetic circuits. The output signal of a Hall sensor which is rotated in a magnetic field with parallel orientation of the field lines correspondingly follows the projection of the field onto the sensitive face of the Hall element of a sinusoidal curve, with the result that given perpendicular orientation of the field with respect to the Hall plane the maximum signal is generated and the gradient at this point is, however, equal to zero so that this characteristic curve range is virtually unusable. Without complex signal processing with a sinusoidal curve, a sufficiently linear profile is obtained only in a range of +/−20° around the zero crossover, while, for example, for the sensing of the throttle valve position a measuring range of >90° is required so that a linear range of at least +/−45° around the zero crossover would be necessary.
A solution of the problem which operates satisfactorily but is at the same time very costly is to linearize the flux profile using iron circuits (see, for example, EP 0 665 416 B1). However, due to the expenditure incurred for the iron circuits the cost advantage of the Hall sensor compared to the principles mentioned at the beginning is for the most part lost again, especially since it is necessary to use for the magnets materials composed of rare earths, for example samarium cobalt, which are very expensive magnetic substances. In addition, it is necessary to take into account the fact that a material with a very low coercivity has to be used for the flux conducting components in order to avoid hysteresis effects when the directions of movement change.
A solution of the type described at the beginning is known from EP 1 069 400 B1. In said document, an output signal, linearized in a relatively large angular range, of a Hall sensor whose surface is located radially and perpendicularly with respect to the rotational axis is obtained through selective influencing of the magnetic field. A problem with such a rotational angle transducer is that positional deviations of the sensor with respect to the magnetic field can influence the signal output in an undesired way, with the result that when mounting the sensor it is necessary to ensure that the Hall sensor is positioned particularly precisely within the magnetic field, and this is not always readily the case, for example when assembling a throttle valve housing. In addition, two redundant channels with as far as possible an identical signal output are often required, and said channels cannot be readily implemented in the solution described in EP 1 069 400 B1 since when two measuring pickups are arranged next to one another there is inevitably a phase offset between the output signals of the Hall sensors.